I didn't realize we were neighbors. You must have added a signature line?
Latex Portland cement mortar in this case is Latex or polymer modified thinset mortar. (A118.4 or A118.11). This and the unmodified thinset are the adhesives. You install the mat according to whatever directions the manufacturer wants. Of course the mat does not cover the entire floor areas. So, your choice on how to proceed. You can shim-coat the rest with modified to get a flat surface, or maybe use a SLC (self leveling cement) product to get it flat. You can also use a patching cement for this. Be sure the leveling products will work over plywood, you'll probably need a primer. The floor has to be flat, it does not have to be level.
Since after the above you will now have a cementitious substrate, Schluter wants you to use an unmodified under Ditra. You then install your tiles over Ditra with a good quality unmodified thinset. (A118.1)
Are you shopping at HD, Lowes, or a real tile store? There are many modified thinsets available, (A118.4 or A118.11), As for a good unmodified, I always use Ditra Set, but it's not so easy for you to find. I believe HD no longer carries any unmod. Lowes carries "MegaBond". It's made by Laticrete for them. It's the only one they carry that can be used either with water for A118.1, or mixed with their latex to make it a modified.
Whatever you do, if you're shopping at the big box, do not pay any attention to what they tell you. Although it could happen....most of the time those nice people have no clue.
TILE GUY - retired- TROY, MI - Consulting by phone/internet - PAYPAL
DITRA Installs - KERDI Watertight-Mold-Free Showers. I have NEVER made a mistake, I thought I did ONCE, but, I was WRONG!A+ BBB rating - est. 1987 - over 50 yrs. exp.